The invention relates to in-band signaling, and in particular to optical cross-connect switches providing an in-band signaling capability. The invention finds application to micro-electromechanical systems (MEMS), but it is not so limited.
The optical cross-connect switch promises to become a key element of fiber-optic networks. Steering elements in the optical switch are used to direct beams of light from input fibers to the desired output fibers (or equivalently, optical waveguides). The amount of optical power coupled through a given connection is a function of a) the amount of power present in the input fiber, b) insertion losses at the fibers, lenses, and mirrors, c) coupling loss due to mode radii mismatches and longitudinal errors, and d) coupling loss due to errors in beam alignment as effected by the steering elements.
Previous optical switches have not capitalized on the loss due to errors in beam alignment as a means of intentionally modulating the power emerging from the output fibers en route to certain other elements of the optical network. A fraction of signal power can be utilized to generate a set of commands to signal other elements downstream along the optical path. This command set can be encoded as a sequence of events expressed by different optical power levels, discrete frequencies, or their combination. Such in-band signals travel along the same optical channels available for data. One representative use is to optically add a key to the data channel through the “send” optical switch allowing the “receive” optical switch to use the key to determine what optical connection should be made for the incoming data. In an all-optical network employing optical switches in conjunction with power-level-sensitive elements, such as optical amplifiers and multi-wavelength systems, the signaling function currently executed using dedicated elements can potentially be absorbed by the optical switch, thereby minimizing or eliminating the need for expensive post-switch signaling sources.
In-band signaling techniques can be implemented with a single mirror actuated with at least one actuator used as a beam-steering element; the power loss is governed by the mirror deflecting angle in at least one axis away from a set of angles that maximize coupling.
In prior art, such as found in U.S. Pat. Nos. 6,711,340; 6,484,114 and 6,556,285 of the assignee of the present invention Glimmerglass Networks, Inc., techniques were disclosed for optical beam alignment of cross-connect switches and similar optical devices using a constant amplitude dithering technique. The subject matter of these patents is incorporated by reference herein for all purposes. Although the power-loss mechanism is similar to the loss mechanism employed in the present context, the prior art is not applicable to the general case of using two or more steering elements, nor to the specific case of an in-operation optical switch utilizing prescribed motions of the steering elements that prohibit the methods of the prior patents. The need exists for a technique for optical modulation that can be effected using the steering elements of an in-operation optical switch without deleterious impact on the switching functions.
It has been shown in U.S. Pat. Nos. 6,556,285 and 6,484,114 that four coordinate errors can be detected using synchronous detection. The four orthogonal oscillations, dithers, of the output errors are produced through associated motion of the mirrors as defined by a kinematic mapping between an actuator space and a coupling space. Each mirror in the optical path is actuated with a pair of orthogonal, in the x and y directions, angular displacement commands. These imposed angular displacement commands that effect dither are in addition to the nominal mirror steering commands that center the beams on the mirrors. The four unknown alignment errors are detected by demodulating the logarithm of the power with the four time-orthogonal dither mode variable signals and averaging over the minimum time period that the four dither cycles repeat. It has been shown that the effects of the four dithers on the logarithm of the power are separable and proportional to the alignment errors that can be individually minimized. In the prior art, the objective has been to maintain a constant power at the output fiber with minimal optical loss of a connection using dithering. In contrasts, as will be explained, an object of the present invention is to excite an in-band signal, or a modulated signal with unique frequency signatures.
The need exists for an in-band signaling technique for optical modulation that can be effected using the steering elements of an in-operation optical switch without deleterious impact on the switching functions and which is robust in the presence of noise and variations of in-line optical power.